1. Field of the Invention
The present invention relates to a norbornene-based copolymer for photoresist, a preparation method thereof, and a photoresist composition comprising the same. More particularly, the present invention relates to the norbornene-based copolymer for photoresist, which is synthesized by use of a 5-norbornene-2-alkane-1,3-dione having pKa of about 5 (pKa of carboxylic acid) to about 10 (pKa of phenolic group) and its derivatives as essential comonomers and, optionally, ketal compound, which is a derivative of the above 5-norbornene-2-alkane-1,3-dione and the photoresist composition comprising the same.
2. Description of the Related Art
In general, photoresist for producing micropatterns must exhibit a low light-absorption to laser light of 193 nm wavelength, an excellent etching resistance, heat resistance and adhesion. And photoresists, which permit development in tetramethylammonium hydroxide (TMAH) solution, used recently in the fabrication of semiconductor devices, are advantageous for production cost. However, it is very difficult to produce photoresists to meet such various requirements.
To date, much effort has been made to obtain resin with high transparency to laser light of 193 nm wavelength and excellent etching resistance. For example, acrylic acid derivatives and methacrylic acid derivatives which have been widely used as base monomers of argon fluoride (ArF) exposure photoresist material have good transparency. However, they have disadvantage of poor etching resistance, such that the film produced therefrom cannot be served as masks.
As one of measures for improving etching resistance of ArF exposure photoresist material, there is an approach to introduce alicyclic olefin units to the side chain and the main chain of acryl-based copolymers. Introduction of alicyclic olefin units improves etching resistance of ArF exposure photoresist material, but is problematic in that, due to high acidity (pKa is about 5) of carboxyl group, which is a soluble functional group, the copolymer is dissolved faster than KrF exposure photoresist material having phenolic group (pKa is about 10) as a soluble functional group in alkaline developer solution, resulting in a pattern having an inferior profile, such as top loss profile. Top loss profile means the roundness of top of patterns while forming micropatterns.
Further more, carboxyl group has strong adhesion to TMAH, so that this method is problematic in that it may make patterns not tight due to swelling and if developer is used undiluted, the unexposed part is also dissolved.
Thus, to solve these problems, a study to introduce a ketal group of keto ester having the pKa more than 11 to ArF exposure photoresist have been conducted. This method reduces sensitivity to TMAH solution, resulting in the slowness of dissolving rate.
However, this method is problematic in that the pKa of keto ester substituted is too high to cause development in TMAH solution; as the mole fraction of keto ester substituted increases, the hydrophilicity of a resin is reduced and the adhesion of the resin to substrate is weakened, which make the formation of micropatterns difficult.
To cope with these problems, the present invention provides a photoresist copolymer having pKa of about 5 (pKa of carboxyl group) to about 10 (pKa of phenolic group) and a photoresist composition comprising the same, which is stable and can form excellent pattern profile by virtue of high resolution and remarkable substrate adhesion.
It is a feature of the present invention to provide a novel 5-norbornene-2-alkane-1,3-dione derivative, represented by the following Formula 1b: 
wherein R1 is C1-12 linear, branched or cyclic alkyl group;
R2 is hydrogen atom or C1-6 linear, branched or cyclic alkyl group;
R3, R4 and R5 are independently hydrogen atom or C1-6 linear or branched alkyl group, or R1 and R5 are bonded with each other to form a cyclic diketone;
R6 and R7 are independently C1-6 alkyl group, or R6 and R7 are bonded with each other to form a ring;
p is an integer of 0 to 6; and
m is 0 or 1.
It is another feature of the present invention to provide a preparation method of a 5-norbornene-2-alkane-1,3-dione derivative having the structure which is represented by the above Formula 1b, comprising transketalizing the 5-norbornene-2-alkane-1,3-dione which is represented by the following Formula 1a with a cyclic ketal or cyclic acetal compound in the presence of a acid catalyst in an organic solvent: 
wherein R1 is C1-12 linear, branched or cyclic alkyl group;
R2 is hydrogen atom or C1-6 linear, branched or cyclic alkyl group;
R3, R4 and R5 are independently hydrogen atom or C1-6 linear or branched alkyl group, or R1 and R5 are bonded with each other to form a cyclic diketone;
p is an integer of 0 to 6; and
l is 0 or 1.
It is still another feature of the present invention to provide a norbornene-based copolymer for photoresist, represented by the following Formula 3: 
wherein R1 is C1-12 linear, branched or cyclic alkyl group;
R2 is a hydrogen atom or C1-6 linear, branched or cyclic alkyl group;
R3, R4 and R5 are independently hydrogen atom or C1-6 linear or branched alkyl group, or R1 and R5 are bonded with each other to form a cyclic diketone;
R6 and R7 are independently C1-6 alkyl group, or R6 and R7 are bonded with each other to form a ring;
p is an integer of 0 to 6;
R8 and R9 are independently hydrogen atom, C1-10 linear or branched alkyl group, xe2x80x94(CH2)qxe2x80x94C(O)OR10, xe2x80x94(CH2)qxe2x80x94OR10, xe2x80x94(CH2)qxe2x80x94C(O)R10, xe2x80x94(CH2)qxe2x80x94OC(O)R10, xe2x80x94(CH2)qxe2x80x94OC(O)OR10 or xe2x80x94(CH2)qxe2x80x94C(O)OCH2OR10, in which R10 is a hydrogen atom or C1-10 linear, branched or cyclic alkyl group; q is an integer of 0 to 6; or R8 and R9 are bonded with each other to form a ring;
a, b and c independently satisfy 0.01xe2x89xa6a/(a+b+c)xe2x89xa60.30, 0xe2x89xa6b/(a+b+c)xe2x89xa60.50 and 0.20xe2x89xa6c/(a+b+c)xe2x89xa60.99; and
l, m and n is each independently 0 or 1.
It is still another feature of the present invention to provide a preparation method of the copolymer of the Formula 3, which comprises the following steps:
(a) dissolving Pd(II) catalyst in a solvent selected from the group consisting of water, an organic solvent and a mixture thereof; and
(b) adding the 5-norbornene-2-alkane-1,3-dione compound of the above Formula 1a, norbornene derivatives of the following Formula 2 and, optionally, derivatives of the above 5-norbornene-2-alkane-1,3-dione of the above Formula 1b as comonomers to the catalyst-dissolved solution and reacting the resultant mixture under non-activating gas stream or reduced pressure: 
wherein R1 is C1-12 linear, branched or cyclic alkyl group;
R2 is a hydrogen atom or C1-6 linear, branched or cyclic alkyl group;
R3, R4 and R5 are independently hydrogen atom or C1-6 linear or branched alkyl group, or R1 and R5 are bonded with each other to form a cyclic diketone;
p is an integer of 0 to 6; and
l is 0 or 1, 
wherein R1 is C1-12 linear, branched or cyclic alkyl group;
R2 is a hydrogen atom or C1-6 linear, branched or cyclic alkyl group;
R3, R4 and R5 are independently hydrogen atom or C1-6 linear or branched alkyl group, or R1 and R5 are bonded with each other to form a cyclic diketone;
R6 and R7 are independently C1-6 alkyl group, or R6 and R7 are bonded with each other to form a ring;
p is an integer of 0 to 6; and
m is 0 or 1, and 
wherein R8 and R9 are independently hydrogen atom, C1-10 linear or branched alkyl group, xe2x80x94(CH2)qxe2x80x94C(O)OR10, xe2x80x94(CH2)qxe2x80x94OR10, xe2x80x94(CH2)qxe2x80x94C (O)R10, xe2x80x94(CH2)qxe2x80x94OC(O)R10, xe2x80x94(CH2)qxe2x80x94OC(O)OR10 or xe2x80x94(CH2)qxe2x80x94C(O)OCH2OR10, in which R10 is a hydrogen atom or C1-10 linear, branched or cyclic alkyl group, q is an integer of 0 to 6, and R8 and R9 can be bonded with each other to form a ring; and
n is o or 1.
It is still another feature of the present invention to provide a composition for photoresist comprising (a) the copolymer according to the Formula 3, (b) a photo acid generator, and (c) a solvent which can dissolve the components (a) and (b).
It is still another feature of the present invention to provide a preparation method of micropatterns, which comprises the following steps:
(a) coating a substrate with a photoresist composition to form a photoresist film;
(b) prebaking the coated substrate on a hot plate;
(c) exposing the prebaked film through a mask with radiation having a wavelength of 250 nm or less;
(d) postbaking the exposed film on a hot plate; and
(e) developing the postbaked film.
Other features and advantages of the present invention will become apparent to those skilled in the art from the following detailed description. It is to be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not limitation. Many changes and modifications within the scope of the present invention may be made without departing from the spirit thereof, and the invention includes all such modifications.